Don't clear " register 'o' or 'O' on empty line

An 'o' or 'O' on an empty line would cause the registerChangeHandler in
VimState to receive a change of newText '', oldText ''. The old text of ''
would be put in the " register, wiping out anything that had been yanked
previously. This made it impossible to do e.g. y, y, <move to another, empty,
line>, o, p